DENIERS SAY:

Global warming happens first, carbon dioxide rises afterward.

SCIENCE SAYS:

Carbon dioxide is a control knob on our climate. The more we pollute, the warmer it gets.

Our climate has warmed over the last 130 years because of carbon pollution from human activities. At other times in the Earth's history, long before humans started burning fossil fuels, the climate warmed for other reasons, such as changes in the Earth's orbit. As the temperature increased in the past, oceans also released more carbon dioxide because warm water holds less carbon dioxide than cold water. This led to more warming, and even more carbon dioxide as a result. The bottom line is this: The fact that global warming happened for different reasons hundreds of thousands of years ago does not change the fact that today, human activity is responsible for climate change. We are putting 35 billion tons of carbon pollution into the atmosphere every year. And scientists agree that is the reason our planet is now warming.

Additional info from Skeptical Science

Earth’s climate has varied widely over its history, from ice ages characterized by large ice sheets covering many land areas, to warm periods with no ice at the poles. Several factors have affected past climate change, including solar variability, volcanic activity and changes in the composition of the atmosphere. Data from Antarctic ice cores reveals an interesting story for the past 400,000 years. During this period, CO2 and temperatures are closely correlated, which means they rise and fall together. However, based on Antarctic ice core data, changes in CO2 follow changes in temperatures by about 600 to 1000 years, as illustrated in Figure 1 below. This has led some to conclude that CO2 simply cannot be responsible for the current global warming.

This statement does not tell the whole story. The initial changes in temperature during this period are explained by changes in the Earth’s orbit around the sun, which affect the amount of seasonal sunlight reaching the Earth’s surface. In the case of warming, the lag between temperature and CO2 is explained as follows: as ocean temperatures rise, oceans release CO2 into the atmosphere. In turn, this release amplifies the warming trend, leading to yet more CO2 being released. In other words, increasing CO2 levels become both the cause and effect of further warming.